http2: don't log about timeouts reading client preface on new connections

Updates golang/go#18776

Change-Id: I7568f779e2b86c72c54c8744c08cc02988dde55b
Reviewed-on: https://go-review.googlesource.com/79498
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>

http2/server.go

@@ -652,7 +652,7 @@ func (sc *serverConn) condlogf(err error, format string, args ...interface{}) {
 	if err == nil {
 		return
 	}
-	if err == io.EOF || err == io.ErrUnexpectedEOF || isClosedConnError(err) {
+	if err == io.EOF || err == io.ErrUnexpectedEOF || isClosedConnError(err) || err == errPrefaceTimeout {
 		// Boring, expected errors.
 		sc.vlogf(format, args...)
 	} else {
@@ -897,8 +897,11 @@ func (sc *serverConn) sendServeMsg(msg interface{}) {
 	}
 }
 
-// readPreface reads the ClientPreface greeting from the peer
-// or returns an error on timeout or an invalid greeting.
+var errPrefaceTimeout = errors.New("timeout waiting for client preface")
+
+// readPreface reads the ClientPreface greeting from the peer or
+// returns errPrefaceTimeout on timeout, or an error if the greeting
+// is invalid.
 func (sc *serverConn) readPreface() error {
 	errc := make(chan error, 1)
 	go func() {
@@ -916,7 +919,7 @@ func (sc *serverConn) readPreface() error {
 	defer timer.Stop()
 	select {
 	case <-timer.C:
-		return errors.New("timeout waiting for client preface")
+		return errPrefaceTimeout
 	case err := <-errc:
 		if err == nil {
 			if VerboseLogs {